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Which of the following sets of ordered pairs define a functions?
a. {(3,2), (-3,6), (3,-2), (-3,-6)}
b. {(1,2), (2,6), (3,-2)(4,-6)}
c. {(2,2), (2,3), (2,4), (2,-9)}
d. {(4,4), (-3,4), (4,-4), (-3,-4)
Answer plzzz?!​